Minimization of CaF2 spherical surface deformation caused by annealing

Abstract. We explore causes and possible techniques to minimize shape deformation of high-precision monocrystalline calcium fluoride optical surface formed during thermal annealing. Due to the anisotropic mechanical properties of calcium fluoride crystals, machining of lens shapes introduces defects into the crystal lattice. The thermal annealing thus leads to activation of the processes of recovery, resulting in the formation of characteristic surface structures causing both shape error and increased microroughness. The surface deformation can be gradually minimized by thermal treatment of the optical element and subsequent polishing steps to the order of units of nanometers, so they do not represent a fundamental problem for optical performance.

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